The invention relates to the positive effect on gastro-intestinal health of food, food ingredients or nutritional supplements with particular compositions of arabinoxylans. Arabinoxylan (AX), also referred to as pentosan, is a major constituent in the cell wall of many plant species. For instance in cereal grains, AX occurs at 5-10% of dry weight of the grains. In general, AX from cereals consists of a backbone of beta-(1-4)-linked D-xylopyranosyl residues (xylose), some of which are mono- or disubstituted with alpha-L-arabinofuranosyl residues (arabinose) (lzydorczyk and Biliaderis,1995). The ratio of arabinose to xylose (AIX ratio or average degree of arabinose substitution) in cereal AX ranges from 0.10 to over 1.0, depending on tissue and plant species. In addition, more minor substituents can be attached to the xylose residues such as acetyl, alpha-glucuronyl, alpha-4-O-methylglucuronyl, galacturonyl, xylosyl, rhamnosyl, galactosyl, or glucosyl side chains, or short oligosaccharide side chains (Izydorczyk and Biliaderis, 1995; Andersson and Aman, 2001). Hydroxycinnamic acids, mainly ferulic acid, and to a lesser extent dehydrodiferulic acid, p-coumaric acid, or sinapic acid, are present as substituents as well, and they are generally linked to the C—(O)-5 position of terminal arabinose units (lzydorczyk and Biliaderis, 1995; Andersson and Aman, 2001). AX in cereals occurs in two forms, a water extractable form, also referred to as WE-AX, and a form that is water-unextractable (WU-AX) most likely due to covalent or non-covalent interactions with neighbouring AX molecules and other cell wall components such as proteins, cellulose or lignin (Andersson and Aman, 2001; Courtin and Delcour, 2002). In wheat grains, the AX present in aleurone and seed coat tissues are mainly water-unextractable AX (WU-AX) and have a low NX ratio (about 0.1 to 0.4), while AX from the pericarp tissues are WU-AX with a high AIX ratio (about 1.0 to 1.3) (Andersson and Aman, 2001; Barron et al. 2007). The AX in the endospermic tissues of wheat are either WU-AX or WE-AX with an intermediate AIX ratio (about 0.5 to 0.7) (lzydorczyk and Biliaderis, 1995; Andersson and Aman, 2001).
Part of the WU-AX in cereal grains can be solubilised by low dose endoxylanase treatment. The enzyme-solubilized AX (ES-AX) and alkali-solubilised (AS-AX) have similar physicochemical properties as WE-AX (Courtin and Delcour, 2002). We will here refer to the group of WE-AX, AS-AX and ES-AX as water-soluble AX (WS-AX).
Prebiotics are compounds, usually oligosaccharides, that can not be digested by enzymes of the upper gastro-intestinal tract but are fermented selectively by some types of intestinal bacteria in the large intestine (Gibson and Roberfroid, 1995; Roberfroid, 1988; Van Loo, 2004). Ingestion of prebiotics causes a shift in the composition of the intestinal bacterial population, typically characterised by a relative increase in Lactobacillus and Bifidobacterium species. This shift in the intestinal microbiota is associated with improved overall health, reduced gut infections, better absorption of minerals, and suppression of colon cancer initiation (Van Loo, 2004; Macfarlane et al. 2006).
Fermentation of prebiotics by colonic bacteria gives rise to production of short chain fatty acids (SCFA) such as acetate, propionate, butyrate and lactate, which act as electron sinks of respiration in the anaerobic environment of the gut. The presence of SCFA in the intestines contributes to a lower pH, a better bio-availability of calcium and magnesium, and inhibition of potentially harmful bacteria (Teitelbaum and Walker, 2002; Wong et al. 2006). Among the SCFA , butyrate appears to be of greatest interest as butyrate is a preferred energy source for colonocytes (Roediger, 1982), stimulates colon epithelial cells, thereby increasing the absorptive capacity of the epithelium (Topping and Clifton, 2001), and inhibits the growth of colonic carcinoma cells, both in vitro and in vivo (Scheppach et al 1995). The cancer-suppressing properties of dietary fibres appear to correlate with their ability to generate butyrate upon colonic fermentation (Perrin et al. 2001).
The selective stimulation by prebiotics of certain colonic bacteria, such as Lactobacilli and Bifidobacteria, which typically use saccharolytic pathways to fuel their energy needs, is in some cases paralleled by suppression of protein fermentation in the colon (van Nuenen et al. 2003; De Preter et al. 2004; Geboes et al. 2005). Reduced protein fermentation in the colon is a desired outcome, as the amino acid degradation pathways in bacteria result in the production of potentially toxic catabolites such as ammonia, amines, phenols, indoles, and thiols, some of which have been implicated in bowel cancer (Bone at al 1976; Johnson, 1977; Visek 1978) and in exacerbation of diseases such as ulcerative colitis (Ramakrishna et al 1991).
Preparations of xylo-oligosaccharides (XOS, oligosaccharides consisting of β-1,4-linked D-xylopyranosyl units) with predominance of oligosaccharides with a degree of polymerisation (DP) of 2-3 (xylobiose and xylotriose), have been shown to cause a significant increase in the level of Bifidobacteria and SCFA in the faeces and caecum of rats (EP 0265970B1; Campbell at al., 1997; Hsu et al., 2004), and the colon of humans (Okazaki et al., 1990). Such xylobiose-rich XOS preparations also suppress early symptoms of chemical-induced colon carcinogenesis in rats (Hsu et al., 2004) and enhance the absorption of calcium in the colon (Toyoda at al., 1993). Experiments described in WO2006/002495 have provided evidence that arabinoxylan-derived oligosaccharides, also referred to as arabinoxylan-oligosaccharides or AXOS, with an intermediate average DP (avDP) ranging from 5 to 50 have better prebiotic properties than AXOS with higher avDP, and are less sweet than AXOS preparations with a lower avDP. Addition of such AXOS preparations to the diet causes a significant increase in the number of Bifidobacteria present in the caecum of chickens, caecum of rats, and faeces of humans (WO2006/002495).
Prebiotics, including AXOS, typically are water-soluble oligosaccharides, which can be readily incorporated into a wide range of food products without noticeably affecting their taste and texture. Therefore, prebiotics are generally considered as particularly suitable ingredients in the preparation of processed foods low in dietary fibre. Indeed, the addition of prebiotics allows to confer to such food certain of the health benefits associated with the presence of dietary fibre, without altering their appealing appearance, taste and texture. On the other hand, dietary fibre rich foods such as whole grain foods or bran enriched foods are typically not supplemented with prebiotic oligosaccharides.
In the context of the present invention it was shown that the water-unextractable arabinoxylan, such as contained in whole grain and bran-enriched foods, is a particularly suitable substrate for the formation of butyric acid in the large intestine. Moreover, it was surprisingly found that a combined consumption of water-unextractable arabinoxylan and arabinoxylan-oligosaccharides had a synergistic effect on the production of butyric acid in the large intestine. This finding indicates that regardless of their high dietary fibre content it is beneficial to supplement existing foods containing substantial amounts of water-unextractable arabinoxylans, such as whole grain or bran enriched foods with arabinoxylan-oligosaccharides. On the other hand, the synergistic effect of arabinoxylan-oligosaccharides and water-unextractable arabinoxylans on intestinal butyrate production allows to prepare foods containing water-unextractable arabinoxylans, which in combination with the arabinoxylan-oligosaccharides provide upon ingestion desirable levels of butyrate in the large intestine, while having a pleasant taste and texture. So in a first aspect the present invention relates to nutritional compositions, including food products, containing suitable levels of water-unextractable arabinoxylans and arabinoxylan-oligosaccharides, which upon ingestion provide a desirable intestinal production of butyrate. Further, it was observed that the consumption of a nutritional composition comprising both water-soluble arabinoxylans and arabinoxylan-oligosaccharides stimulated the production of butyrate, while strongly suppressing the protein fermentation in the large intestine. So in a second aspect the present invention relates to nutritional compositions comprising both water-soluble arabinoxylans and arabinoxylan-oligosaccharides, which upon ingestion provide for production of butyrate and inhibition of protein fermentation in the large intestine.